1. Field of the Invention
The present invention relates to the field of Hall effect thrusters.
2. Description of Related Art
The invention relates more particularly to a steerable-thrust Hall effect thruster having an annular channel, an anode, an injection circuit, a magnetic circuit, and a cathode. The annular channel is defined by two concentric walls with a central axis, having an open end and a closed end, and it includes an upstream segment beside the closed end that is subdivided by radial walls into a plurality of separate compartments. The anode is situated at the closed end of the annular channel. The injection circuit is suitable for injecting a propulsion gas, e.g. such as xenon, into the compartments of the annular channel, and it includes at least one individual flow rate regulator device for each compartment. The magnetic circuit is suitable for generating a magnetic field at the open end of the annular channel. The cathode is situated on the outside at the open end of the annular channel.
The terms “upstream” and “downstream” in the present context are defined relative to the normal flow direction of the propulsion gas along the axis defined by the central axis of the annular channel.
Typically, when such a Hall effect thruster is in operation, electrons emitted by the cathode and attracted towards the anode at the bottom of the annular channel are trapped by the magnetic field in spiral trajectories between the two walls, thus forming a virtual cathode grid. Electrons escaping from this magnetic enclosure towards the anode come into collision with atoms of propulsion gas injected into the annular channel by the injection circuit, thereby creating an ionized plasma.
The positive ions of the plasma are accelerated by the electric fields that exist between the anode and the virtual cathode grid formed by the cloud of electrons trapped by the magnetic field at the open end of the annular channel. Since the mass of such a positive ion is much greater than the mass of an electron, the trajectory of the ions is hardly affected by the magnetic field. The ions in this plasma jet are finally neutralized downstream from the magnetic field by electrons emitted from the cathode or that have been produced by the ionization of the plasma.
Hall effect thrusters have begun to be used in attitude and orbit control systems (AOCSs) for space vehicles, and in particular in AOCSs for geostationary satellites. In that function, Hall effect thrusters have the advantage of enabling the attitude and/or the position of the vehicle to be controlled accurately while requiring significantly smaller mass and less complexity than conventional systems using inertial devices, such as for example reaction wheels, in combination with chemical thrusters for desaturation purposes.
Typically, the thrust force from a Hall effect thruster is not steerable, which can make it necessary to use a plurality of Hall effect thrusters simultaneously in order to obtain a thrust force in a desired direction in order to change the pointing direction and/or the position of the space vehicle. This implies in particular an electrical power supply circuit for the thrusters that is quite complex. Alternatively, a Hall effect thruster mounted on a pivot mount for steering the thrust force of the thruster is described, for example in the article “Inmarsat 4F1 plasma propulsion system initial flight operations” (IECP-2005-082) described by H. Grey, S. Provost, M. Glogowski, and A. Demaire in the 29th International Electric Propulsion Conference in 2005, Princeton, USA. Nevertheless, such a pivot mount presents a considerable amount of mechanical complexity, and it requires moving parts that always run the risk of jamming in the very severe environment of a space vehicle.
To mitigate those drawbacks, U.S. Pat. No. 5,845,880 proposes a Hall effect thruster in which thrust can be steered by a last magnetic stage that is subdivided into individually activatable sectors. The orientation of the thrust can thus be varied by varying the magnetic field, which presents drawbacks for maintaining the magnetic enclosure around the entire perimeter of the open end of the annular cathode, and thus for maintaining the virtual cathode grid. Furthermore, the electrical power supply to the final magnetic stage of variable power also adds a certain degree of complexity to the thruster.
The article “Performance and lifetime assessment of a thrust steering device for the PPS® 1350 Hall effect plasma thruster” by O. Duchemin, M. Saverdi, and D. Estublier, published in the 2008 Space Propulsion Conference, May 5-8, 2008 at Heraklion, Greece, and in the article “Performance modeling of a thrust vectoring device for Hall effect thrusters” published in the “Journal of Propulsion and Power”, Vol. 25, No. 5, September-October 2009, tests are described on a steerable-thrust Hall effect thruster similar to that of U.S. Pat. No. 5,845,880, but including, in addition to a final magnetic stage subdivided into individually activatable sectors, a plurality of propulsion gas injection nozzles distributed in the annular channel, with individual flow rate regulation in order to obtain varying and non-uniform distribution of the gas that is injected into the annular channel. Nevertheless, in those articles, the steering of the thrust by the non-uniform flow rates of gas injected into the annular channel is described as being relatively ineffective, and it is even discouraged in view of the additional complexity of the flow rate regulator devices.
European patent application EP 1 021 073 A1 likewise describes a steerable-thrust Hall effect thruster having a plurality of propulsion gas injection nozzles distributed in the annular channel with individual flow rate regulation. In addition, in that thruster, compartments are formed in the annular channel by radial walls. Nevertheless, in that document, the non-uniform distribution of gas flow rates in the annular channel is proposed for the purpose of moving the thrust axis laterally without changing its orientation. The magnetic circuit has a final stage with an inner pole that is offset axially upstream relative to the outer pole so as to concentrate the ionized jet of propulsion gas. As in U.S. Pat. No. 5,845,880, the thrust is steered by a non-uniform magnetic field, the final magnetic stage also being subdivided into individually activatable sectors.